1. Field of the Invention
The present invention relates to a laser cutting apparatus.
2. Description of the Related Art
A cutting and machining process, for a metal or resinous material and using a laser beam, has been widely conducted. A laser apparatus for a cutting process generally includes a laser oscillator, as a source of a laser beam, an optical system (or a light guide system) for transmitting and collecting the laser beam onto a material to be processed (i.e., a workpiece), a mechanical driving system for scanning the spots to be irradiated with the beam on the workpiece, a control system for controlling the laser oscillator or the mechanical driving system, and auxiliary apparatuses such as a coolant supply apparatus, an assist gas feeding apparatus, etc.
In general, a laser cutting apparatus of this type is considered to have a higher machining capability when the collecting properties (i.e., a beam quality) of a laser beam are superior. In fact, in the case where a thin metallic material such as a metal sheet is cut, as a conventional main application, with a laser cutting apparatus using a CO2 laser, it is known that the higher the collecting properties of the laser beam, the faster the metal sheet can be cut and the better the surface quality provided by the cutting process.
A laser beam typically emerges from a laser oscillator as a generally parallel beam, and thus has good collecting properties. There may be a case, however, wherein the laser beam cannot be focused, depending upon the energy distribution thereof, into a spot at the diffraction limit. In other words, depending upon the quality of the laser beam, the spot size may vary even when the diffusion angle and the diameter of the beam incident on a collective or condenser lens are not changed. This is due to the collecting properties of the laser beam, and the higher the collecting properties, the smaller the possible spot size.
An M2 value is often used as an index for evaluating the collecting properties of a laser beam. The index M2 is typically defined by a formula:M2=π×(dm)2/(4×λ×Zr)wherein “λ” is a wavelength of the laser beam; “dm” is a minimum beam diameter of the laser beam in a predetermined optical-path range including a focal point of a collective lens; and “Zr” is a distance (or a Rayleigh range) between a first position on an optical axis, at which the minimum beam diameter “dm” is established, and a second position on the optical axis, at which a beam diameter “√2×dm” is established, in the laser beam in the predetermined optical-path range.
FIG. 10 is a view showing an example of the transition of the beam diameter of a laser beam collected by a collective or condenser lens, in the vicinity of a focal point. The horizontal axis represents a position Z (mm) in the direction of the optical axis of the collective lens with the position of the center of the collective lens taken as Z=0, and the vertical axis represents the beam diameter d (mm) of the laser beam at position Z. A point • represents an actually measured value and, by interpolation between several points, it is found that, in the illustrated example, the distance Zr between “the first position defining the minimum beam diameter dm (=about 0.22 mm)” and “the second position defining the beam diameter of √2×dm (=about 0.31 mm)” is about 4 mm.
As can be seen from the above formula of definition, a smaller M2 value indicates smaller spot size and higher collecting properties. The theoretical minimum value of M2 is 1 and, at this value, a laser beam theoretically has the best collecting properties.
Conventionally, a cutting process using a laser beam is mainly applied to the cutting of a thin metal sheet with a thickness of generally 10 mm or less, and the required M2 value for this application is less than 2.8. Typically, a laser beam having M2 value in the vicinity of 2 is employed. In recent years, however, with increasing power output of laser oscillators, it is highly required to cut thick plates of 20 to 30 mm or more in thickness. There is a problem, when such a thick plate is to be cut using a laser, of the difficulty in adequately supplying an assist gas to a cutting groove.
In a cutting operation using a laser beam, in general, the laser beam emerging from a processing nozzle of a processing apparatus is collected so that the surface of a workpiece is irradiated with the laser beam, so as to melt a minute region of the workpiece at an extremely high temperature, and an assist gas is injected from the same processing nozzle, coaxially with the laser beam and at a predetermined pressure and flow rate, so as to locally remove the molten material of the workpiece by physical and chemical interaction with the assist gas.
For example, when a steel plate is to be cut, oxygen in particular is used as the assist gas, and the heat generated in a combustion reaction with the oxygen is also utilized in melting the steel material. As the thickness of the plate increases, the oxygen assist gas supplied to the surface of the steel plate is exhausted midway in the thickness direction so that, near the back surface of the steel plate, the cutting becomes difficult due to a deficiency of oxygen. If, at this time, the pressure of the supplied oxygen gas is increased to increase the amount of supply, the flow rate of the assist gas is excessively increased so that an anomalous combustion (or a self-burning phenomenon) takes place at the front surface of the steel plate. As a result, surface quality of the cut section may be deteriorated, or dross may be accumulated to degrade the function of the products.
Also, in the case where nitrogen or argon gas, that does not generate heat from a combustion reaction, is used as the assist gas, if the thickness of the workpiece is increased, the flow rate tends to be reduced midway in the thickness direction of the workpiece, and the power for blowing away the molten metal, etc. tends to become inadequate. In the case of non-metallic material, the function of the assist gas, as a cooling gas to suppress carbonization or excessive melting, tends to be impaired by the decrease in the flow rate.
In order to deal with these problems, countermeasures, such as proper selection of nozzle diameter, distance from the nozzle to the surface of the workpiece, the shape of the nozzle, etc., or proper distribution of flow rate in the radial direction of the nozzle (by adopting a so-called double nozzle), have been taken. Other measures are also known, in which, when a thick plate is to be cut, a collective lens having a focal length longer than that of a collective lens typically used for cutting a thin plate may be used to perform the cutting process by a portion of the collected laser beam having a larger spot size, so that the cutting width of the workpiece is increased to increase the amount of supplied assist gas in the direction of plate thickness. Similarly, it is also known that the focal position of the collective lens can be adjusted to adjust the spot size or control the cutting width.
However, an optimum design is generally difficult with the technique of controlling the flow of the assist gas by suitably designing the nozzle shape, etc. With the technique of adjusting the focal length or focal position of the collective lens, there is a limit to the thickness of the workpiece to obtain an expected result.
On the other hand, as is described in Japanese Unexamined Patent Publication No. 6-218565 (JP-A-6-218565) and Japanese Unexamined Patent Publication No. 2002-118312 (JP-A-2002-118312), it is known that the collecting properties of a laser beam can be controlled so as to be able to cut both thin plates and thick plates. In the methods disclosed in these Patent Documents, the cutting of a thin plate is conducted with the laser oscillation in TEM00 mode (the so-called Gaussian mode: M2=1.0), and the cutting of a thick plate is conducted with the laser oscillation in TEM01* mode (the so-called ring mode: M2=about 1.7: * denotes a calculation in polar coordinates). In TEM01* mode, as compared to TEM00 mode, the thermal load at the center of the collective lens is small, so that a temperature rise at the center of the collective lens due to absorption of laser beam, and hence the degradation of a beam collecting properties and the variation of the focal position due to associated deformation of the lens shape and of the distribution of refractive index, can be effectively suppressed.
When above-described method is applied, for example, to the cutting of the steel plates with oxygen as assist gas, however, the expected result can only be obtained for thicknesses in the range of 12 mm to 16 mm, and the advantage of the ring mode over Gaussian mode can hardly be recognized at thickness greater than 20 mm.